1. Field of the Invention
Apparatuses consistent with the present invention relate to a polar transmitter which increases a modulation rate using a multi-phase generator, and more particularly, to a polar transmitter which is suitable for a multi-mode and a multi-band by increasing a modulation rate using a multi-phase generator.
2. Description of the Related Art
A typical spread spectrum transmitter uses sine waves or pulses as a carrier to convey information, by increasing the sine waves or the pulses to a certain frequency. In order to perform the increase in frequency, the transmitter requires components which up-convert the carrier from a baseband to the certain frequency.
To up-convert the carrier to the certain frequency, the transmitter can adopt a superheterodyne principle which up-converts from the baseband to a certain frequency via an intermediate band and a direct conversion principle which converts the baseband directly to a certain frequency.
A superheterodyne transmitter requires an intermediate frequency (IF) voltage controller oscillator (VCO) for generating frequencies of the IF band, a radio frequency (RF) VCO for generating frequencies of the RF band, an IF phase locked loop (PLL) and a RF PLL which fix the frequencies generated at the VCOs not to be affected by external factors, an IF up-mixer which up-converts the carrier of the baseband to the IF band of the frequencies generated at the VCOs, and a RF up-mixer which up-converts the carrier of the IF band to a certain frequency band. In addition, the superheterodyne transmitter requires a surface acoustic wave (SAW) filter for eliminating image signals and a power amplifier (PA) having good linearity. Such a superheterodyne transmitter is disadvantageous in that a number of circuit parts complicate the circuitry and their coordination.
A direct conversion transmitter requires a VCO, a PLL, an up-mixer, and a PA. As such, the number of the circuit parts of the direction conversion transmitter is smaller than that of the superheterodyne transmitter. Still, the good linearity of the PA is required and a separate filtering means should be provided at the back end of the PA to meet the noise level required by the GSM standard. Additionally, noise due to DC components is disadvantageous and the signal quality is deteriorated due to mismatch of an in-phase (I) signal and a quadrature (Q) signal.
To overcome the shortcomings of the superheterodyne transmitter and the direct conversion transmitter, a polar transmitter has been suggested.
The polar transmitter separates data of I signal and Q signal to an amplitude component and a phase component for processing. The polar transmitter includes a modulator 10, a PLL 20, a VCO 30, a PA 40, an amplitude control loop 50, and a phase control loop 60, as shown in FIG. 1.
The modulator 10 receives the I signal and the Q signal and separates them to the amplitude component and the phase component. The PLL 20 generates a control signal so that the VCO 30 outputs a carrier of a frequency having a phase that matches to the input phase component.
The PA 40 receives the generated carrier from the VCO 30 and an amplitude control signal from the amplitude control loop 50 and then outputs a carrier. The carrier produced from the PA 40 has an intended phase and an intended amplitude.
The amplitude control loop 50 generates the amplitude control signal to be applied to the PA 50 by processing the amplitude component separated at the modulator 10. The amplitude control loop 50 analyzes the amplitude of the carrier output from the PA 40 and provides the amplitude control signal for the amplitude control to the PA 40 according to the amplitude analysis.
The phase control loop 60 analyzes the phase of the carrier output from the PA 40 and provides a signal for controlling the phase to the PLL 20 according to the analysis.
The related art polar transmitter utilizes the PLL 20 to generate the carrier, and the PLL 20 has a feedback circuit so that the VCO 20 generates the carrier having the exact frequency and phase. The feedback circuit of the PLL 20 operates every time the amplitude and the phase of the carrier are modulated. At every phase modulation, it takes a time to compare the phases in the feedback circuit. The related art polar transmitter further includes two more feedback circuits of the amplitude control loop 50 and the phase control loop 60, in addition to the feedback circuit of the PLL 20. Accordingly, the entire modulation for the carrier output takes a long time.
Meanwhile, with advances and diversity of communication standards for the radio communications, a transceiver for radio communications is required which can not only operate in different modes but also comply with one or more standards or frequency bands. For instance, a new radio communication transceiver is required to support CDMA 1X and/or General Packet Radio Service (GPRS), global system for mobile communications (GSM), and Wideband Code Division Multiple Access (WCDMA).
To support the multi-band capability, one or more reference oscillators are required in a single transceiver. To this end, in the related art, two separate PLLs 20 and a switch between them are provided. However, such a design increases the size and the cost and degrades the design efficiency.
To address these disadvantages, a multi-mode radio communication transceiver requires one or more VCOs 30 and a PLL 20 that can operate in the multiple frequencies to support the diverse standards. Yet, the time delay due to the feedback circuit of the PLL 20 is not suitable for the PLL 20 in a wideband system.
Therefore, what is needed is a circuit that can operate at a high rate and promptly modulate a new frequency even when changing to a different mode.